1. Field of the Invention
The present invention relates to image-forming devices.
2. Description of the Prior Art
An image-forming device employing an electrophotographic process, such as laser printer, forms an image on a recording medium while transferring a toner image formed on a photosensitive drum surface onto the recording medium.
Constant voltage control, in which a constant transfer voltage is maintained, and constant current control, in which a constant transfer current is maintained, are well utilized in order to transfer the toner image formed on the photosensitive drum surface onto the recording medium. On the other hand, the resistance of the recording paper often changes due to humidity changes. When the constant voltage control is utilized, the transfer current is decreased if the resistance is increased. As a result, faulty transfers often occur due to the insufficient transfer current. Accordingly, it is preferable that the constant current control is utilized.
Meanwhile, current flows directly from the transfer roller to the photosensitive drum, when the transfer roller is contact with the photosensitive drum. When the resistance of the recording medium is reduced, the current that flows directly from the transfer roller to the photosensitive drum is increased without charging the recording paper. As a result, faulty transfer occurs due to the insufficient transfer current. In order to resolve this problem, Japanese patent unexamined publication 2002-202671 discloses a technique that assumes that the transfer current flowing directly from the transfer roller to the photosensitive drum is increased when the transfer voltage drops below a prescribed value, and switches from the constant current control to the constant voltage control in order to increase the transfer current.
However, the transfer current flowing directly from the transfer roller to the photosensitive drum is not is increased necessary when the transfer voltage drops below the prescribed value. Therefore, Japanese patent unexamined publication 2002-202671 cannot take desired amount of current to the photosensitive drum necessarily.